the analogue of magnetic force; and a region in which the masses of the wheels are largo corresponds to a region of high magnetic permeability. The indiarubber bands of FitzGerald's model correspond to the medium in which Maxwell's vortices were embedded; and a strain on the bands represents dielectric polarization, the line joining the tight and slack sides of any band being the direction of displacement. A body whose specific inductive capacity is large would be represented by a region in which the elasticity of the bands is feeble. Lastly, conduction may be represented by a slipping of the bands on the wheels.
Such a model is capable of transmitting vibrations analogous to those of light. For if any group of wheels be suddenly set in rotation, those in the neighbourhood will be prevented by their inertia from immediately sharing in the motion; but presently the rotation will be communicated to the adjacent wheels, which will transmit it to their neighbours; and so a wave of motion will be propagated through the medium. The motion constituting the wave is readily seen to be directed in the place of the wave, i.e. the vibration is transverse. The axes of rotation of the wheels are at right angles to the direction of propagation of the wave, and the direction of polarization of the bands is at right angles to both these directions.
The elastic bands may be replaced by lines of governor balls:[1] if this be done, the energy of the system is entirely of the kinetic type.[2]
Models of types different from the foregoing have been suggested by the researches of Helmholtz and W. Thomson on vortex-motion. The earliest attempts in this direction, however, were intended to illustrate the properties of ponderable matter rather than of the luminiferous medium. A vortex existing in a perfect fluid preserves its individuality throughout all changes,
